Colorectal cancer is the second most common form of cancer in the United States, but controversy exists over the feasibility, benefits, and methods of screening. We conducted a large-scale, community-based screening program to determine the cost of screening for colorectal cancer. Based on detailed cost analysis, we arrived at a cost of $15,233 per case of colon cancer diagnosed through mass screening, and $7611 per polyp discovered. These figures are much higher than those previously reported in less detailed studies. Other studies currently under way will determine whether morbidity and mortality are decreased by screening. Our data add the information necessary to determine the cost-effectiveness of mass screening for colon cancer, and will be useful in designing future strategies for the secondary prevention of colorectal cancer.